I needed a quick way to resize images in batches, and was pleased to find that Preview has that functionality. Open the whole batch of images in Preview; the images will all open in the same window with thumbnails visible in the sidebar.
Select all of the thumbnails in the sidebar (with Command-A, or click the first and then click the last while holding Shift). Next, from the Tools menu, choose Adjust Size. You’ll be presented with a dialog containing options to resize to all the common 4:3 and 16:9 screen sizes, as well as the option to define custom dimensions. Images are scaled proportionately and resampled by default.
Save the resized images, and you’re done! Preview will prompt you to save any changes if you attempt to close the window.